Variable reluctance rotation sensors are employed in applications where rotational speed must be sensed. In automotive applications, these types of sensors are commonly used within anti-lock braking systems and/or traction control systems which require determination of the rotational speed of the automobile wheels. In the past, these variable reluctance sensors have generally operated by measuring the change in the total magnetic flux level of the main flux loop within the sensor.
However, in U.S. Pat. No. 5,023,546 to Pawlak et al and assigned to the assignee of this patent application, a variable reluctance rotation sensor was disclosed which measures the change in the flux distribution rather than the change in the magnetic flux level. This sensor (also referred to as the Pawlak sensor) contains a magnetic toothed wheel which has teeth evenly spaced by slots around its periphery, wherein the magnetic toothed wheel is rotatably supported with respect to a fixed magnetic pickup assembly.
The magnetic pickup assembly consists of two permanent magnets and a magnetic flux member having a multi-turn electrical coil wound thereon, the magnetic flux member being disposed between the two magnets. The assembly extends adjacent to the rotatable toothed wheel to provide flux loops that enclose only some of the coil turns. The pair of permanent magnets each have north and south poles, with the similar pole of each magnet being adjacent the toothed wheel. The permanent magnets are separated from each other circumferentially around the periphery of the toothed wheel by an arc at least equal to the arc between one of the teeth and a non-adjacent slot. Therefore, the permanent magnets are circumferentially spaced so that when one is aligned with a tooth of the wheel, the other is aligned with a non-adjacent slot, i.e., the magnets are off-set. Hence, as the toothed wheel rotates, the two permanent magnets are alternately and opposingly aligned with teeth and slots. The magnetic flux member, which has the multi-turn electrical coil wound thereon, joins the other of the poles of each permanent magnet is a series opposed relationship, and extends between the permanent magnets in close proximity to the toothed wheel to establish separate, oppositely directed flux loops for each of the permanent magnets along the magnetic flux member. The flux loops are spatially determined by the position of the teeth and slots as they rotate adjacent the magnetic flux member between the permanent magnets. Thus, the flux loops fluctuate circumferentially across the wound coil with the passing of the alternating teeth and slots as the wheel rotates. Correspondingly, an electrical signal is generated in the electrical coil by the variation of flux linkages in individual coil turns as the toothed wheel rotates.
In the Pawlak sensor, because of this off-set configuration of the permanent magnets, the changes in the spatial distribution of the magnetic flux due to the passing teeth and slots in the region between the magnets, with consequent changes in flux linkages to individual coil turns, greatly increase the rate of change in the flux distribution at low rotational speeds. Accordingly, the Pawlak sensor is more sensitive to the magnetic permeability change at these low rotational speeds, as compared to conventional sensors, and thereby exhibits enhanced performance.
However, the magnetic flux member within the Pawlak sensor is generally odd shaped due to the configuration of the sensor and placement of the permanent magnets, and therefore requires considerable machining in order to produce its unique shape. Because of this extensive amount of machining, the machined magnetic flux member is generally highly mechanically stressed. These stresses reduce the magnetic properties of the flux member, in particular its magnetic permeability. This loss in magnetic permeability hinders the sensor of its full sensing potential.
In the past, annealing operations which were designed to alleviate the stresses within the machined magnetic flux member generally provided only limited enhancements to the magnetic permeability of the flux member, resulting in only about a five to ten percent increase in the sensor signal. Such marginal improvements in permeability add little to conventional sensors because they are relatively insensitive to permeability changes, since they measure mostly the change in total flux level and accordingly operate at high magnetomotive forces and high magnetic flux levels. This marginal improvement in permeability to the magnetic circuit is not sufficient to justify the added processing costs associated with annealing of the flux member or the increased time of manufacture for the sensor and are therefore generally not employed.
However, the Pawlak sensor operates differently with its off-set magnets by measuring the change in spatial distribution of the flux through large air gaps along the flux member. Thereby during operation, the magnetomotive force level of the permanent magnets is lower, and the resulting magnetic flux distribution is characterized by lower flux density levels with potentially higher magnetic circuit permeability, as compared to conventional sensors. However, the overall performance of the sensor is more sensitive to the level of magnetic permeability within the magnetic circuit, including the magnetic flux member.
Thus, it would be desirable to provide a means for improving the permeability of the magnetic circuit within the Pawlak sensor, in particular the permeability of the machined magnetic flux member, so as to maximize the sensing capabilities of this variable reluctance sensor having off-set permanent magnets.